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Indium-free Perovskite TCOs Could Save Costs

Monday, January 4th, 2016


By Ed Korczynski, Sr. Technical Editor

Lei Zhang, et al. from Pennsylvania State University—with collaborators from Rutgers University and University of Toledo—have found two new families of transparent conductive oxides (TCO) based on “correlated” electrons in ternary oxides of vanadium. From reported first principles, the co-authors are confident they will find many other correlated materials that behave like strontium vanadate (SrVO3) and calcium vanadate (CaVO3), which could make flat panel displays (FPD) and photovoltaic (PV) modules more affordable.

The correlation relies on strong electron–electron interactions resulting in an enhancement in the carrier effective mass. Both SrVO3 and CaVO3 demonstrate high carrier concentration (>2.2 ×1022 cm−3), and have low screened plasma energies (<1.33 eV). The Figure shows that there is a transparency trade-off in using these new TCOs, since at nominal 10nm thickness they are more than twice as opaque as Indium tin oxide (ITO).

Optical transmission of free standing conductive oxide films at 550nm wavelength, accounting for reflection and interference, and averaged over the range of the visible spectra. (Source: Nature Materials)

ITO has been the dominant TCO used in FPD manufacturing, but the price of indium metal has varied over the range of $100-1000/kg in the last 15 years. Consequently, industry has long searched for a TCO made of less expensive and less variable direct materials. Currently vanadium sells for ~$25/Kg, while strontium is even cheaper. Lei Zhang, lead author of the Nature Materials article ( and a graduate student in assistant professor Roman Engel-Herbert’s group, was the first to recognize the application.

“I came from Silicon Valley where I worked for two years as an engineer before I joined the group,” says Zhang. “I was aware that there were many companies trying hard to optimize those ITO materials and looking for other possible replacements, but they had been studied for many decades and there just wasn’t much room for improvement.” Engel-Herbert and Zhang have applied for a patent on this technology.

The U.S. Office of Naval Research, the National Science Foundation, and the Department of Energy funded this R&D. “Now, the question is how to implement these new materials into a large-scale manufacturing process,” said Engel-Herbert. “From what we understand right now, there is no reason that strontium vanadate could not replace ITO in the same equipment currently used in industry.”

Electrons flow like a liquid

Correlated oxides are defined as metals in which the electrons flow like a liquid, unlike conventional metals such as copper and gold in  which electrons flow like a gas. “We are trying to make metals transparent by changing the effective mass of their electrons,” Engel-Herbert says. “We are doing this by choosing materials in which the electrostatic interaction between negatively charged electrons is very large compared to their kinetic energy. As a result of this strong electron correlation effect, electrons ‘feel’ each other and behave like a liquid rather than a gas of non-interacting particles. This electron liquid is still highly conductive, but when you shine light on it, it becomes less reflective, thus much more transparent.”

In the November 2007 issue of the prestigious Physical Review B (DOI:  10.1103/PhysRevB.76.205110), F. Rivadulla et al. reported on “VO: A strongly correlated metal close to a Mott-Hubbard transition.” Vanadium oxide (VO) has a rocksalt cubic crystal structure, and displays strongly correlated metallic properties with non-Fermi-liquid thermodynamics and an unusually strong spin-lattice coupling. The structural and electronic simplicity of 3D monoxides provides a basic understanding of highly correlated electron systems, while this new work with 2D ternary oxides is inherently more complex.

One positive aspect of the more complex perovskite structure of SrVO3 and CaVO3 is that it provides for intriguing device integration possibilities with other functional perovskite materials. PV devices based on thin-films of complex perovskites have demonstrated excellent photon-electron conversion efficiencies in labs, but commercial manufacturing has so far been limited by the lack of an inexpensive TCO that can be integrated into a moisture barrier. The templating effect of underlayers could allow for faster deposition of more ideal SrVO3.


Research Alert: September 3, 2014

Wednesday, September 3rd, 2014

A new, tunable device for spintronics

Spin-charge converters are important devices in spintronics, an electronic which is not only based on the charge of electrons but also on their spin and the spin-related magnetism. Spin-charge converters enable the transformation of electric into magnetic signals and vice versa. Recently, the research group of Professor Jairo Sinova from the Institute of Physics at Johannes Gutenberg University Mainz in collaboration with researchers from the UK, Prague, and Japan, has for the first time realized a new, efficient spin-charge converter based on the common semiconductor material GaAs.

Comparable efficiencies had so far only been observed in platinum, a heavy metal. In addition, the physicists demonstrated that the creation or detection efficiency of spin currents is electrically tunable in a certain regime. This is important when it comes to real devices. The underlying mechanism, that was revealed by theoretical works of the Sinova group, opens up a new approach in searching and engineering spintronic materials. These results have recently been published in the journal Nature Materials.

Making use of electron spin for information transmission and storage, enables the development of electronic devices with new functionalities and higher efficiency. To make real use of the electron spin, it has to be manipulated precisely: it has to be aligned, transmitted and detected. The work of Sinova and his colleagues shows, that it is possible to do so using electric fields rather than magnetic ones. Thus, the very efficient, simple and precise mechanisms of charge manipulation well established in semiconductor electronics can be transferred to the world of spintronic and thereby combine semiconductor physics with magnetism.

Now, Sinova and his colleagues have shown that gallium-arsenide (GaAs), a very common and widely used semiconductor material, can be an as efficient spin-charge converter as platinum, even at room temperature, which is important for practical applications. Moreover, the physicists have demonstrated for the first time that the efficiency can be tuned continuously by varying the electric field that drives the electrons.

The reason for this – as theoretical calculations of the Sinova group have shown – lies in the existence of certain valleys in the conduction band of the semiconductor material. One can think of the conduction band and its valleys as of a motor highway with different lanes, each one requiring a certain minimum velocity. Applying a higher electric field enables a transition from one lane to the other.

Since the spin-orbit coupling is different in each lane, a transition also affects the strength of the spin-hall effect. By varying the electric field, the scientists can distribute the electron spins on the different lanes, thus varying the efficiency of their spin-charge converter.

By taking into account the valleys in the conduction band, Sinova and his colleagues open up new ways to find and engineer highly efficient materials for spintronics. Especially, since current semiconductor growth technologies are capable of engineering the energy levels of the valleys and the strength of spin-orbit coupling, e.g. by substituting Ga or As with other materials like Aluminum.

Copper shines as flexible conductor

By turning instead to copper, both abundant and cheap, researchers at Monash University and the Melbourne Centre for Nanofabrication have developed a way of making flexible conductors cost-effective enough for commercial application.

“Aerogel monoliths are like kitchen sponges but ours are made of ultra fine copper nanowires, using a fabrication process called freeze drying,” said lead researcher Associate Professor Wenlong Cheng, from Monash University’s Department of Chemical Engineering.

“The copper aerogel monoliths are conductive and could be further embedded into polymeric elastomers – extremely flexible, stretchable materials – to obtain conducting rubbers.”

Despite its conductivity, copper’s tendency to oxidation and the poor mechanical stability of copper nanowire aerogel monoliths mean its potential has been largely unexplored.

The researchers found that adding a trace amount of poly(vinyl alcohol) (PVA) to their aerogels substantially improved their mechanical strength and robustness without impairing their conductivity.

What’s more, once the PVA was included, the aerogels could be used to make electrically conductive rubber materials without the need for any prewiring. Reshaping was also easy.

“The conducting rubbers could be shaped in arbitrary 1D, 2D and 3D shapes simply by cutting, while maintaining the conductivities,” Associate Professor Cheng said.

The versatility extends to the degree of conductivity. “The conductivity can be tuned simply by adjusting the loading of copper nanowires,” he said. “A low loading of nano wires would be appropriate for a pressure sensor whereas a high loading is suitable for a stretchable conductor.”

Affordable versions of these materials open up the potential for use in a range of new-generation concepts: from prosthetic skin to electronic paper, for implantable medical devices, and for flexible displays and touch screens.

They can be used in rubber-like electronic devices that, unlike paper-like electronic devices, can stretch as well as bend. They can also be attached to topologically complex curved surfaces, serving as real skin-like sensing devices, Associate Professor Cheng said.

In their report, published recently in ACS Nano, the researchers noted that devices using their copper-based aerogels were not quite as sensitive as those using gold nanowires, but had many other advantages, most notably their low-cost materials, simpler and more affordable processing, and great versatility.

Competition for graphene

A new argument has just been added to the growing case for graphene being bumped off its pedestal as the next big thing in the high-tech world by the two-dimensional semiconductors known as MX2 materials. An international collaboration of researchers led by a scientist with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has reported the first experimental observation of ultrafast charge transfer in photo-excited MX2 materials. The recorded charge transfer time clocked in at under 50 femtoseconds, comparable to the fastest times recorded for organic photovoltaics.

“We’ve demonstrated, for the first time, efficient charge transfer in MX2 heterostructures through combined photoluminescence mapping and transient absorption measurements,” says Feng Wang, a condensed matter physicist with Berkeley Lab’s Materials Sciences Division and the University of California (UC) Berkeley’s Physics Department. “Having quantitatively determined charge transfer time to be less than 50 femtoseconds, our study suggests that MX2 heterostructures, with their remarkable electrical and optical properties and the rapid development of large-area synthesis, hold great promise for future photonic and optoelectronic applications.”

Wang is the corresponding author of a paper in Nature Nanotechnology describing this research. The paper is titled “Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures.” Co-authors are Xiaoping Hong, Jonghwan Kim, Su-Fei Shi, Yu Zhang, Chenhao Jin, Yinghui Sun, Sefaattin Tongay, Junqiao Wu and Yanfeng Zhang.

MX2 monolayers consist of a single layer of transition metal atoms, such as molybdenum (Mo) or tungsten (W), sandwiched between two layers of chalcogen atoms, such as sulfur (S). The resulting heterostructure is bound by the relatively weak intermolecular attraction known as the van der Waals force. These 2D semiconductors feature the same hexagonal “honeycombed” structure as graphene and superfast electrical conductance, but, unlike graphene, they have natural energy band-gaps. This facilitates their application in transistors and other electronic devices because, unlike graphene, their electrical conductance can be switched off.

“Combining different MX2 layers together allows one to control their physical properties,” says Wang, who is also an investigator with the Kavli Energy NanoSciences Institute (Kavli-ENSI). “For example, the combination of MoS2 and WS2 forms a type-II semiconductor that enables fast charge separation. The separation of photoexcited electrons and holes is essential for driving an electrical current in a photodetector or solar cell.”

In demonstrating the ultrafast charge separation capabilities of atomically thin samples of MoS2/WS2 heterostructures, Wang and his collaborators have opened up potentially rich new avenues, not only for photonics and optoelectronics, but also for photovoltaics.

“MX2 semiconductors have extremely strong optical absorption properties and compared with organic photovoltaic materials, have a crystalline structure and better electrical transport properties,” Wang says. “Factor in a femtosecond charge transfer rate and MX2 semiconductors provide an ideal way to spatially separate electrons and holes for electrical collection and utilization.”

Wang and his colleagues are studying the microscopic origins of  charge transfer in MX2 heterostructures and the variation in charge transfer rates between different MX2 materials.

“We’re also interested in controlling the charge transfer process with external electrical fields as a means of utilizing MX2 heterostructures in photovoltaic devices,” Wang says.

This research was supported by an Early Career Research Award from the DOE Office of Science through UC Berkeley, and by funding agencies in China through the Peking University in Beijing.

The Week in Review: April 11, 2014

Friday, April 11th, 2014

The Semiconductor Industry Association announced that worldwide sales of semiconductors reached $25.87 billion for the month of February 2014, an increase of 11.4 percent from February 2013 when sales were $23.23 billion.

Nanoengineering researchers at Rice University and Nanyang Technological University in Singapore have unveiled a potentially scalable method for making one-atom-thick layers of molybdenum diselenide.

SEMI releases fourth quarter 2013 worldwide photovoltaic equipent market statistics report. Bookings levels improved some in the quarter to reach the highest value since the first quarter of 2012.

Using a laser to place individual rubidium atoms near the surface of a lattice of light, scientists at MIT and Harvard University have developed a new method for connecting particles.

Researchers at Johannes Gutenberg University Mainz (JGU) have achieved a major breakthrough in the development of methods of information processing in nanomagnets.

The Global Semiconductor Alliance (GSA) is celebrating 20 years of industry collaboration this year.

After the successful premier of a program to connect early-stage companies with strategic investors and venture capitalists (VCs) in the U.S., SEMI is expanding the program to Europe as part of SEMICON Europa 2014 in Grenoble, France (October 7-9).

Research News: Nov. 12, 2013

Tuesday, November 12th, 2013

Synaptic transistor learns while it computes

Materials scientists at the Harvard School of Engineering and Applied Sciences (SEAS) have now created a new type of transistor that mimics the behavior of a synapse. The novel device simultaneously modulates the flow of information in a circuit and physically adapts to changing signals.

Exploiting unusual properties in modern materials, the synaptic transistor could mark the beginning of a new kind of artificial intelligence: one embedded not in smart algorithms but in the very architecture of a computer.

In principle, a system integrating millions of tiny synaptic transistors and neuron terminals could take parallel computing into a new era of ultra-efficient high performance.

Synaptic transistorSeveral prototypes of the synaptic transistor are visible on this silicon chip. (Photo by Eliza Grinnell, SEAS Communications.)

While calcium ions and receptors effect a change in a biological synapse, the artificial version achieves the same plasticity with oxygen ions. When a voltage is applied, these ions slip in and out of the crystal lattice of a very thin (80-nanometer) film of samarium nickelate, which acts as the synapse channel between two platinum “axon” and “dendrite” terminals. The varying concentration of ions in the nickelate raises or lowers its conductance—that is, its ability to carry information on an electrical current—and, just as in a natural synapse, the strength of the connection depends on the time delay in the electrical signal.

Structurally, the device consists of the nickelate semiconductor sandwiched between two platinum electrodes and adjacent to a small pocket of ionic liquid. An external circuit multiplexer converts the time delay into a magnitude of voltage which it applies to the ionic liquid, creating an electric field that either drives ions into the nickelate or removes them. The entire device, just a few hundred microns long, is embedded in a silicon chip.

Diamond imperfections pave the way to technology gold

Researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have taken an important step towards unlocking this key with the first ever detailed look at critical ultrafast processes in these diamond defects.

Using two-dimensional electronic spectroscopy on pico- and femto-second time-scales, a research team led by Graham Fleming, Vice Chancellor for Research at UC Berkeley and faculty scientist with Berkeley Lab’s Physical Biosciences Division, has recorded unprecedented observations of energy moving through the atom-sized diamond impurities known as nitrogen-vacancy (NV) centers. An NV center is created when two adjacent carbon atoms in a diamond crystal are replaced by a nitrogen atom and an empty gap.

The next big thing in the energy sector: Photovoltaic generated DC energy

Rajendra Singh, D. Houser Banks Professor in the Holcombe Department of Electrical and Computer Engineering and PhD student Githin F. Alapatt  at  Clemson University, along with  and Akhlesh Lakhtakia, Charles Godfrey Binder (Endowed) Professor in Engineering Science and Mechanics at the Pennsylvania State University, recently examined the most promising types of solar cells to power every home. On October 23, 2013 they published a paper entitled “Making Solar Cells a Reality in Every Home: Opportunities and Challenges for Photovoltaic Device Design” in IEEE Journal of Electron Devices (Volume 1, number 6, June 2013 Issue).

The researchers have proposed a new multi-terminal multi-junction architecture for inexpensive PV electricity generation. Efficiency will exceed the currently feasible 25%. The proposed architecture is based on the use of currently commercial Crystalline solar cells and thin-film solar cells made of materials (such as copper oxide) that are abundant in Earth’s crust. Management of the flux of solar photons through the solar cells is expected to boost efficiency, but the additional manufacturing costs to be incurred thereby remain unknown, according to the researchers.

Prof. Singh says that “the creation of local DC power grids can save power being lost in the transmission and unnecessary conversion from DC to alternating current (AC) and then back to DC.” Most electronic appliances and electric loads operate on DC and by transmitting and converting AC power to DC about 30% of the total power generated is lost. Today, PV electricity generation and distribution on a DC microgrid is the best way to power villages without access to electricity. It is also the best option to replace aging power generation and transmission infrastructure in USA and other developed countries.

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